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Accounting Policies Summary of Significant Accounting Policies (Details) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accounting Policies [Abstract] |  |  |
Percentage of customer revenue | 45.00% | 43.00% | 43.00%
Accounts receivable recorded investment past due days | 90 |  |

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- Definition The disclosure indicates the age when individual accounts are reviewed. + References No definition available. + Details
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- Definition The disclosure indicates the percentage of revenue generated by the largest customers. + References No definition available. + Details
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